The invention relates to internal combustion engines. In particular, the invention pertains to starting a free piston engine by cyclically increasing displacement of a piston that reciprocates due to pressure forces applied by an expanding and compressing air charge and an external periodic force applied by a actuator before ignition occurs.
A free piston internal combustion engine includes one or more reciprocating pistons located in a combustion cylinder. But there is no crankshaft mutually connecting the pistons and causing them to reciprocate when actuated by a starter-alternator, as in a conventional internal combustion engine. In a free piston engine running under normal operation, each piston moves during an expansion stroke in its cylinder in response to forces produced by combustion of an air-fuel mixture in the cylinder. Pressure produced by combustion in one cylinder is used to compress an air-fuel charge in another cylinder. Before combustion occurs while starting the engine, an actuating system is be used to compress the air-fuel charge following the expansion stroke. Motion of the pistons is controlled by a system, which synchronizes piston reciprocation, compression of the air-fuel mixture, and its combustion. Piston displacement and velocity, cylinder pressure, and the compression ratio are monitored and controlled by the system, which periodically corrects deviations from desired, synchronized reciprocation of the pistons.
While starting a free piston engine, the pistons are displaced by a starter-actuator system using hydraulic, pneumatic or electric actuation. Preferably, electric energy is used to actuate the pistons when starting an engine that produces electric output, and hydraulic or pneumatic energy is used to actuate the pistons when starting an engine that produces hydraulic or pneumatic output. When starting a free piston operating under compression ignition, a large compression ratio of the fuel-air charge in the combustion cylinder is required to produce combustion. When conventional engine starting techniques are used, a large magnitude of energy is required to produce the compression ratio required to start the engine, especially under cold starting conditions.
If the pistons are driven entirely by an actuator before combustion while starting the engine, a large magnitude of energy is required to compress the mixture of fuel and air in the combustion chamber, particularly when cold starting a compression ignition free piston engine in cold weather. A technique is required to avoid the need for a large capacity energy source to start the engine.